I was searching around for a way to remove duplicate lines from a text file that weren't necessarily adjacent to one another, while maintaining the line order. I found this solution by BrowserUk:

#! perl -sw
use strict;
my %lines;
#open DATA, $ARGV[0] or die "Couldn't open $ARGV[0]: $!\n";
while (<DATA>) {
    print if not $lines{$_}++;
}

__DATA__
this is a line
this is another line
yet another
and yet another still
this is a line
more
and more
and even more
this is a line
and this
and that
but not the other cos its a family website:)

[download]

It worked for me, but I don't know why. I still don't fully see the magic of hashes. Can someone explain why this works? Specifically the if not $lines{$_}++ structure? I really don't see how incrementing works here. Also, could you replace if not with unless?

Thanks!

In reply to Why does this hash remove duplicate lines by kangaroobin

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