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;