Add some more testcases to this:

m[^(?=.*[a-zA-Z])[\w]+$] and print "$_ : ok" 
    for qw[ 123 123_ 123_a a_ a_123 abc _ a123];
123_a : ok
a_ : ok
a_123 : ok
abc : ok
a123 : ok
[download]

Tweaked: The \d was redundant.