Both, your and Abigail-IIs solution will fail for duplicate elements (although I don't know whether that's a problem or not) :

#!/usr/bin/perl
use strict;
use warnings;

my @list = qw( 1 1 2 );

my %hash;
@hash{@list}=1;

delete $hash{$_} for (1..@list);

print "is sequence\n" unless %hash;
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perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
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