Another approach is to use embedded flags, it is possible to embed all the modifiers except /g and /c

#!/net/perl/5.10.0/bin/perl

use strict;
use warnings;

use 5.010_000;

my $string = 'This is perl, the Perl interpreter';

foreach my $flag ( '', 'i' ) {
    if ( my ($match) = $string =~ /(?$flag:(Perl))/ ) {
        print "$match\n";
    }
}
Perl
perl
__END__
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Update: also doesn't work with /c, add it above.

Have a look at eval. Something like this (untested) should work as you desire. I don't think you need the block eval but do not have the ability to test. eval has security implications.

eval{ eval "(\$output = \$string) =~ s/\$regexp/\$new/$suffix" };
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The outer eval is indeed not needed. Both forms of eval catch exceptions.

As long as $suffix is constrained to /^[xismg]+\z/, there should be no security issues from the eval. There are, however, security issues with regexes, though. The following should be a good *start*:

my $string = ...;
my $regexp = ...;
my $new    = ...;
my $suffix = ...;

my $timeout = 2;

my $output;

$suffix =~ /^[xismg]+\z/
   or die("Bad suffix\n");

my $sop = eval "sub { s/\$regexp/\$new/$suffix }";
   or die("Bad regexp: $@\n");

eval {
   local $SIG{ALRM} = sub { die "Alarm\n" };
   alarm($timeout);
   $sop->() for $output = $string;
   1
} or die("Timeout\n");
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The denial of service isn't completely addressed. Even though the search is limited to $timeout seconds, the user can send requests faster than once a second.

The security risk from bugs in the regexp engine isn't addressed at all.

muy bueno amigo/a

updated regexp s/mode quicktest accordingly