To wit: test.pl

use strict;
use warnings;
use Tie::File;

my $infile = shift;

tie my @lines, 'Tie::File', $infile;
printf "File has %d lines\n",$#lines+1;
print "$_\n" for @lines;

# clobber file
@lines = ();
printf "\nFile has %d lines\n",$#lines+1;
print "$_\n" for @lines;

# insert replacement lines
push @lines, "replacement line here";
push @lines, "another new line";
printf "\nFile has %d lines\n",$#lines+1;
print "$_\n" for @lines;


untie @lines;
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Let's run it.

 $ perl test.pl test.file
File has 4 lines
one
two
three
four

File has 0 lines

File has 2 lines
replacement line here
another new line
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Verify that we have in fact changed the file:

 $ cat test.file
replacement line here
another new line
 $
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Updated for readability