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use warnings;
use strict;
...
    return $string;  # return string with substitution(s)

    }

'1' append undefined
'12' -> '1_2'
...
'1234' -> '1_2_34'
'12345' append undefined
'1x12x123x1234x12345' -> '1x1_2x1_2_3x1_2_34x12345'