Perl's binding operator. (Perl got this from csh.)

=~ is used to specify the scalar variable that is being regexped or otherwise mangled by the expressions m//, s/// or tr///.

Same in simpler terms: The following...

if(/pattern/) {          # Pattern match
  # ...

s/something/other/gi;    # Pattern replace

tr/abc/def/;             # Character transliteration

...all operate on the scalar variable $_ - the default variable of many commands.

But with this operator, we can check other variables just as easily:

if($var =~ /pattern/) {

$var =~ s/something/other/gi;

$var =~ tr/abc/def/;

Now these operators match or modify the variable $var instead.

Return value is typically success/failure status in scalar context (so you can just use if($var =~ /pattern/)... to check if string $var matches the regular expression pattern.)

In list context the return value depends on operation done, but practically, the idea is same. If /g modifier is used, m// returns matched subexpressions ($1, $2, $3...) or, in case there's no parenthesis in the pattern, just list (1) if success. It returns empty list if there's no match. If /g is used, it likewise returns substrings or, if there's no parentheses in the pattern, whole matched strings. (I have been told s/// returns the same in list context. Must check later what tr/// returns...)

There's a related operator, !~, that has negated return value. (In other words, if($var !~ /pattern/)... checks if the pattern doesn't match.)

(Source: perlop)