The use of my or local to create local variables within a subroutine.

 sub max {
	my $max = shift(@_);
	foreach $foo (@_) {
	    $max = $foo if $max < $foo;
	}
	$arg = "fred";   # a "global" variable
	return $max;
    }
 $bestday = max($mon,$tue,$wed,$thu,$fri);  

LOOPS:
Here's how a C programmer might code up a particular algorithm in Perl:

 for (my $i = 0; $i < @ary1; $i++) {
	for (my $j = 0; $j < @ary2; $j++) {
	    if ($ary1[$i] > $ary2[$j]) {
		last; # can't go to outer :-(
	    }
	    $ary1[$i] += $ary2[$j];
	}
	# this is where that last takes me
    }  

Whereas here's how a Perl programmer more comfortable with the idiom might do it:

 OUTER: for my $wid (@ary1) {
    INNER:   for my $jet (@ary2) {
		next OUTER if $wid > $jet;
		$wid += $jet;
	     }
	  }  

BLOCKS and flow-of-control:
Here are three ways to do the same thing:

 SWITCH: {
	if (/^abc/) { $abc = 1; last SWITCH; }
	if (/^def/) { $def = 1; last SWITCH; }
	if (/^xyz/) { $xyz = 1; last SWITCH; }
	$nothing = 1;
    }  

or

 SWITCH: {
	$abc = 1, last SWITCH  if /^abc/;
	$def = 1, last SWITCH  if /^def/;
	$xyz = 1, last SWITCH  if /^xyz/;
	$nothing = 1;
    }  

(That's actually not as strange as it looks once you realize that you can use loop control "operators" within an expression, That's just the normal C comma operator.)

or

 SWITCH: {
	/^abc/ && do { $abc = 1; last SWITCH; };
	/^def/ && do { $def = 1; last SWITCH; };
	/^xyz/ && do { $xyz = 1; last SWITCH; };
	$nothing = 1;
    }  


A common idiom for a switch statement is to use foreach's aliasing to
make a temporary assignment to $_ for convenient matching:

 SWITCH: for ($where) {
		/In Card Names/     && do { push @flags, '-e'; last; };
		/Anywhere/          && do { push @flags, '-h'; last; };
		/In Rulings/        && do {                    last; };
		die "unknown value for form variable where: `$where'";
	    }  

SEARCHING, PATTERN-MATCHING:

m/PATTERN/cgimosx
/PATTERN/cgimosx

    Searches a string for a pattern match, and in scalar context returns true if it succeeds, false if it fails. If no string is specified via the =~ or !~ operator, the $_ string is searched. (The string specified with =~ need not be an lvalue--it may be the result of an expression evaluation, but remember the =~ binds rather tightly.) See also perlre. See perllocale for discussion of additional considerations that apply when use locale is in effect.

    Options are:

    c	Do not reset search position on a failed match when /g is in effect.
    g	Match globally, i.e., find all occurrences.
    i	Do case-insensitive pattern matching.
    m	Treat string as multiple lines.
    o	Compile pattern only once.
    s	Treat string as single line.
    x	Use extended regular expressions.  

Example:
 
    open(TTY, '/dev/tty');
    <TTY> =~ /^y/i && foo();	# do foo if desired

    if (/Version: *([0-9.]*)/) { $version = $1; }

    next if m#^/usr/spool/uucp#;

    # poor man's grep
    $arg = shift;
    while (<>) {
	print if /$arg/o;	# compile only once
    }

    if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))  

REPLACEMENTS:

s/PATTERN/REPLACEMENT/egimosx

    Searches a string for a pattern, and if found, replaces that pattern with the replacement text and returns the number of substitutions made. Otherwise it returns false (specifically, the empty string).

example:
   

    s/\bgreen\b/mauve/g;		# don't change wintergreen

    $path =~ s|/usr/bin|/usr/local/bin|;

    s/Login: $foo/Login: $bar/; # run-time pattern

    ($foo = $bar) =~ s/this/that/;	# copy first, then change

    $count = ($paragraph =~ s/Mister\b/Mr./g);  # get change-count

    $_ = 'abc123xyz';
    s/\d+/$&*2/e;		# yields 'abc246xyz'
    s/\d+/sprintf("%5d",$&)/e;	# yields 'abc  246xyz'
    s/\w/$& x 2/eg;		# yields 'aabbcc  224466xxyyzz'