use strict;
use warnings;

my @email = (
    'someone@example.co.uk',
    'andrew.test@some.company.co.uk',
    'jo@abc.com',
    );

for my $em ( @email ) {
    print "$em - ";

    ( my $at_pos = rindex $em, '@' ) > 0 or die "$em: invalid email address.\n";
    ( my $dot_pos = rindex $em, '.' ) > $at_pos or die "$em: invalid email address.\n";
    tr//./c for substr( $em, 2, $at_pos - 2 ), substr( $em, $at_pos + 3, -( length( $em ) - $dot_pos ) );

    print "$em\n";                                        
    }