My take on this would be to start at the end and work backwords.

#!/usr/bin/perl

use warnings;
use strict;

my $before = [qw(1 2 3 4 5 6 7 8 9 10)];
my $after  = [qw(2 3 4 5 1 6 7 8 9 10)];

my $len = @{$before}-1;

for (reverse @{$before}){
    print $before->[$len], "\t";
    print $after->[$len],  "\n";
    last if $before->[$len] != $after->[$len];
    $len--;
}

#$len++;
print "offset: $len\n";
print "element: $after->[$len]\n";
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output:

---------- Capture Output ----------
> "c:\perl\bin\perl.exe" _new.pl
10    10
9    9
8    8
7    7
6    6
5    1
offset: 4
element: 1
> Terminated with exit code 0.
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