Using the updated problem (numbers starting at 1 in order, only one missing), here's something completely DIFFerent :)

#!/usr/bin/perl -l

# http://perlmonks.org/?node_id=1186862

use strict;
use warnings;
use Algorithm::Diff qw(traverse_sequences);

my @arr = (1..3,5,6,7);

traverse_sequences( \@arr, [ 1 .. @arr + 1 ],
  { DISCARD_B => sub { print "missing: ", 1 + $_[1] } } );
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