$ perl -le '$1=undef;'
Modification of a read-only value attempted at -e line 1.

##</code><code>##

"1234"=~/(\d)/;
print "$1\n";
"1234"=~/34/;
print "$1\n";
print "undef\n" if ! defined $1;

##</code><code>##

1

undef

##</code><code>##

"1234"=~/(x)?34/;
print "undef" if ! defined $1;