my $a = qr/turtle|fish/i;
foo($a);

sub foo{
    my $reg = shift;
    if(ref($reg) eq 'Regexp'){
       my $string = 'fisH';
       print "hello" if ($string =~ $reg);
    }
}
[download]

'/turtle|fish/i' is a Perl expression, not a regular expression. turtle|fish and (?i-xsm:turtle|fish) are (Perl) regular expressions. Since '/turtle|fish/i' is Perl, you'd need eval to execute it. So you need to pass just the regular expression ((?i-xsm:turtle|fish)) or precompile it with qr// (qr/turtle|fish/i).

For example,

$uncompiled_re = '(?i-xsm:turtle|fish)';
mysub($uncomiled_re);

$compiled_re = qr/turtle|fish/i;
mysub($comiled_re);
[download]

Strings get tricky when \ is used, but qr// doesn't. For example,

$uncompiled_re = "(?i-xsm:\\w)";
mysub($uncomiled_re);

$compiled_re = qr/\w/i;
mysub($comiled_re);
[download]

The documented usage is
$string =~ /$re/
whether $re contains a string or a compiled regexp, but I think
$string =~ $re
works in both cases.

The only reason the subroutine must only accept a regex is because that is the requirement imposed by the problem. Without such restriction, the subroutine could also accept a string (in which case, one easy way to validate would likely involve an eval() ).

The code, $string =~ $re ;, will work in both cases:

1. when $re is of type Regexp, as in $re=qr/\w/i,
2. when $re is just a scalar for a string, as in
   • $re = '(?i-xsm:\w)', or
   • $re = '\w';