use strict;
use warnings;

my $string = "string";

if( $string m/([5]?)string/ ){
    print "? inside group: $1\n"; #prints fine
}

if( $string m/([5])?string/ ){
    print "? outside group: $1\n"; #Use of uninitialized value $1 in concatenation (.) or string...
}

return 0;