use strict;

## I've lazily pre-sorted these ;)
my @ars = (
  [ qw/ 1 2 3 4 5 / ],
  [ qw/ 1 2 3 4 6 / ],
  [ qw/ 2 3 4 6 7 / ],
);

for(@ars) {
  print "broken list: @$_\n" and next
    if $_->[0] != 1;
  
  my($i, $n) = 0;
    
  $n = $_->[$i]
    while $i <= $#{$_} and $_->[$i] + 1 == $_->[++$i];
         
  print "woo: @$_\n"
    if $n == $_->[-1];
  print "doh: sequence broke at index ${\($i - 1)} (@$_[$i - 1,$i])\n"
    if $n != $_->[-1];
}

__output__

woo: 1 2 3 4 5
doh: sequence broke at index 3 (4 6)
broken list: 2 3 4 6 7