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use strict;
use warnings;
...
#$val1 5234 prints 5_2_34
#$val1 52   prints 5_2
#$val1 523  prints 5_2_3

for (qw(5234 523 52)) 
{
...
    $string =~ s/$character(\d)$/$1/ if @digits >= 4;
    return $string;
}