If you can work with just pairings then a lookup table is very fast.

my @list =( 10, 5, 13, 3, 15, 1 );
my $total = 16;

my %lookup;
@lookup{@list} = ();

for my $val( @list ) {
    my $need = $total -$val;
  next unless exists $lookup{$need};
    print "Found $val + $need = $total\n";
    delete $lookup{$val};
}
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