I want to open that file for writing in "append" mode.

And just to be different, try this:

#!/usr/bin/perl -wT
use strict;
use Tie::File;

my @ry;

tie @ry,"Tie::File","myfile.txt" or die $!;

push @ry,"stuff";

untie @ry;

exit(0);
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In Perl there is always more than one solution to a problem.... and nobody expects the Perl Inquisition!

update: fixed minor typo pointed out by Happy-the-monk