my $s = 'abc123+';
$s =~ s/[^0-9+]//g;

##</code><code>##

$s =~ s/[^0-9]+//g;

##</code><code>##

$s = 'abc123+';
# remove any non-digit
$s =~ tr/0-9//cd;
# or remove any non-alphanumeric
$s =~ tr/A-Za-z0-9//cd;