You mean you start with /test/smi from some outside source? The only way to use that is to eval it. Of course, that means using eval on something from an outside source, which is a big no-no.

If the user can only use / as the delimiter, then a simple regexp will process the options:

use strict;
use warnings;

# For example, get regexp from command line:
$regepx = $ARGV[0];

# Look for and remove end slashes and modifiers.
$regexp =~ s{^/}         {} or die("Bad input\n");
$regexp =~ s{/([msix]*)$}{} or die("Bad input\n");

my $modifiers_on  = $1;

my $modifiers_off = join '',
                    grep { index($modifiers_on, $_) < 0 }
                    qw( x i s m );

# Add the modifiers:
$regexp = "(?${modifiers_on}-${modifiers_off}:${regexp})";

# Compile the regexp and check for errors:
$regexp = eval { qr/$regexp/ }
   or die("Bad regexp: $@\n");

# Now you can use it:
print($str =~ $regexp ? 'match' : 'no match', "\n");
[download]

It's trickier if you want to support substitutions and the (g)lobal modifier. You're better off asking for these as seperate arguments. (The search string, the replace string and the modifiers.)