use strict;
use warnings;
use 5.010;

my %pics;

while (<DATA>) {
    chomp;
    $pics{$_} = 1;
} 

my @unique_names = keys %pics;

foreach (@unique_names) {
    say;
}
__DATA__
/a/b/img1.jpg
/a/b/img1.jpg
/a/b/img2.jpg
/x/y/img2.jpg
/x/y/img2.jpg
/x/y/img3.jpg

##</code><code>##

/a/b/img2.jpg
/a/b/img1.jpg
/x/y/img3.jpg
/x/y/img2.jpg