A bit shorter:

use strict;
use warnings;

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

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

    $em =~ /^(\w[\w.])(.*)(\@\w\w)(.*)(\.\w+)$/;
    $em = $1 . '.' x length($2) . $3 . '.' x length($4) . $5;

    print "$em\n";
}
[download]

Or, you may like this:

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

    print "$em\n";                                        
    }
[download]