Given:

__DATA__
Success

Invalid string!

...
    {
    if (/^[a-zA-Z0-9]{6,20}\z/) {
...

prints:

Invalid string: >Success
<
Invalid string: >
<
Invalid string: >Invalid string!
<

and

...
    {
    chomp;
    
    if (/^[a-zA-Z0-9]{6,20}\z/) {
...

prints:

Success: >Success<
Invalid string: ><
Invalid string: >Invalid string!<