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col1 col2 col3 col4 col5 1234 5678 9012 3456 7890
col1 col2 cYl2 col3 col4 col5 1234 5678 5Y78 9012 3456 7890
use strict; use warnings; ... $line =~ s/(.{11}).(.*)/$1Y$2/; print $line."\n"; }