You could try it this way: (for large files)
** Updated to id::1382's suggestion... (is that documented somewhere? ... ***

sub perform {
        # do per line processing to avoid out-of-memory error.
        open( FILE, "< ./file.txt" ) or die "Can't open : $!";
        while (my $line = <FILE>) {
            processLine(chomp($line));
        }
        close FILE;
}

sub processLine {
        my $line=shift;
        # etc.
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