A regex is a double-quote-ish expression, so variables are interpolated, including dereferencing references. Any block of code can be stuck in place of a reference (it should return a reference, in order to dereference properly). So you can execute any code you want within a regex or a string. The result of the block will be dereferenced and interpolated into the string (here I return a ref to an empty string, so the pattern isn't changed):

It will be executed each time the regex is looked at (compiled), even if the match doesn't get that far. For example:

for (qw{dog pig cat horse}) {
    /.o${print "$_ gets printed\n";\''}/ and print "$_ matched\n";
}
__END__
dog gets printed
dog matched
pig gets printed
cat gets printed
horse gets printed
horse matched
[download]

Contrast that with (?{}), which executes arbitrary code if the expression matched up to that point (and does not insert anything into your pattern):

for (qw{dog pig cat horse}) {
    /.o(?{print "$_ gets printed\n"})/ and print "$_ matched\n";
}
__END__
dog gets printed
dog matched
horse gets printed
horse matched
[download]

And finally (??{}), which acts the same, but does insert into your pattern:

for (qw{dog pig cat horse}) {
    /.o(??{print "$_ gets printed\n"; 'g'})/ and print "$_ matched\n";
}
__END__
dog gets printed
dog matched
horse gets printed
[download]

The latter two are "experimental" features, but ref interpolation is not.