use warnings;
use strict;
while (my $line = <DATA>){
    chomp $line;
    $line =~ s/(.{5})(.{4})/$1 . $2 . ' ' . substr($2, 0, 1) . 'Y' . substr($2, 2, 2)/e;
    print $line . "\n";
}

__DATA__
col1 col2 col3 col4 col5
1234 5678 9012 3456 7890

##</code><code>##

col1 col2 cYl2 col3 col4 col5
1234 5678 5Y78 9012 3456 7890