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

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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.

In reply to Re^3: in file insert character in every row on nth position by Discipulus
in thread in file insert character in every row on nth position by ukhare

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