Here's a naive solution based on the special variable $., which holds the number of lines already read from the last filehandle: 
use strict;
open INPUT, "<myfile.txt" or die "Could not open input file: $!\n";
open OUTPUT, ">output.txt" or die "Could not open output file: $!\n";
my $count=0;
while (<INPUT>)              # Loop over lines of file
{ 
next unless ($. > 2);        # Ignore first two lines
s/do my/replacement/ if ($. == 3);
print OUTPUT $_;             # Print to output file.
}
close INPUT;
close OUTPUT;

[download]
When given the input file: 
Line 1
Line 2
Line 3 do my
Line 4
it returns the output file: 
Line 3 replacement
Line 4
Have fun exploring the wacky world of Perl!

CU
Robartes-

In reply to Re: Deleting and replacing lines in a file by robartes
in thread Deleting and replacing lines in a file by Dobro

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