Gimme a string: a
Gimme a RegEx: a(
Unmatched ( in regex; marked by <-- HERE in m/a( <-- HERE / at .. line .. <STDIN> line 2.

##</code><code>##

use strict;
use warnings;

print "Gimme a string: ";
my $str = <STDIN>;
chomp $str;
print "Gimme a RegEx: ";
my $pattern = <STDIN>;
chomp $pattern;

my $rex;
{ # a block to localize $@
 local $@;
 # eval the regex 
 eval{ $rex = qr/$pattern/ };
 # die if errors
 die "error compiling regex!" if $@;

} # end of the localizing block


print $str =~ /$rex/ ? "Yes!" : "No.";


##</code><code>##

perl -E "say 'match' if 'ec' =~ /$ARGV[0]/" "ec(?{print 'EVAL!';})"

Eval-group not allowed at runtime, use re 'eval' in regex m/ec(?{print 'EVAL!';})/ at -e line 1.