your error must coming from elsewhere..

perl -e "if(length($ARGV[0])==11){print $ARGV[0], (substr($ARGV[0],-10
+,6))}" 1234567890_
1234567890_234567
[download]

Anyway you presented few stroke of your code and it is impossible to tell if $11 come from the 11th capturing group of a regex or is very bad choice for a variable name: it is a special variable.

the code above acts as you expect?

L*