in reply to Re^2: in file insert character in every row on nth position
in thread in file insert character in every row on nth position

Let Perl (well B::Deparse ) eplain it to you:
perl -pi -e "s/^(.{13})/$1:/;" -MO=Deparse BEGIN { $^I = ""; } LINE: while (defined($_ = <ARGV>)) { s/^(.{13})/$1:/; } continue { die "-p destination: $!\n" unless print $_; } -e syntax OK

L*

There are no rules, there are no thumbs..
Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.