my $text = 'perl monks';

my $query = qr{
    (?<M>monk)
    (?(<M>)(?{do_something_different()}))
    |
    (?<P>perl)
    (?(<P>)(?{do_something()}))
}x;

while($text =~ /$query/gi){};

sub do_something{
    print "found perl\n";
}

sub do_something_different{
    print "found monk\n";
}