is it possible to accomplish this in one s/// command?
Yes, but it is hideously ugly, and you shouldn't do it this way (update... do it BrowserUk's way): 
use warnings;
use strict;
while (my $line = <DATA>){
    chomp $line;
    $line =~ s/(.{5})(.{4})/$1 . $2 . ' ' . substr($2, 0, 1) . 'Y' . s
+ubstr($2, 2, 2)/e;
    print $line . "\n";
}

__DATA__
col1 col2 col3 col4 col5
1234 5678 9012 3456 7890

[download]
prints: 
col1 col2 cYl2 col3 col4 col5
1234 5678 5Y78 9012 3456 7890

[download]

In reply to Re: Search pattern within pattern in a single substitute command by toolic
in thread Search patern within pattern in a single substitute command by shobhit

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