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  1. or download this
    $ perl -le '$1=undef;'
Modification of a read-only value attempted at -e line 1.
  2. or download this
    "1234"=~/(\d)/;
print "$1\n";
"1234"=~/34/;
print "$1\n";
print "undef\n" if ! defined $1;
  3. or download this
    1

undef
  4. or download this
    "1234"=~/(x)?34/;
print "undef" if ! defined $1;