(?{ CODE }) will execute any perl code when the regex engine tries to match it. So for example:

my $string = "Bar";
$sting =~ /Foo(?{ print "Found a Foo\n" })|Bar(?{ print "Found a Bar\n
+" })|Baz(?{ print "Found a Baz\n" })/;
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Should print out 'Found a Bar'.

It's documented in the perlre man page under 'Extended Patterns'.