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use strict;
use warnings;
...
my $file = shift or die "usage: $0 file\n";
tie my @lines, 'Tie::File', $file or die "error: $!";
print $lines[$#lines];   # this is the last line in the file

use Fcntl 'O_RDONLY';    # add this line
#   ... as before, then change the tie line as shown below
tie my @lines, 'Tie::File', $file, mode => O_RDONLY or die "error: $!"
+;
#   ... as before