perlsub. Even perlintro! (Spend 20 minutes reading perldoc perlintro before your next question.)

One thing to note: Probably the best way to pass a regular expression to your subroutine is to pass a regexp object, obtained using the qr// operator described in perlop. Example:

sub test_match {
  my( $regexp, $string ) = @_;
  return scalar( $string =~ $regexp );
}

my $string = "Hello world!\n";
print $string if test_match( qr/hello/i, $string );
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