I guess you user would seperate their addresses with a space or a comma. I would check on that.

Something like:

#!/usr/bin/perl -w

use strict;

sub is_single
{
        my $email = shift;
        # remove spaces from beginning and end
        $email =~ s/^\s*|\s*$//g;
        # check for characters invalid in an emailaddress
        return $email=~/[\s,]/?0:1;
}

# prints 1
print is_single('hpotter@hotmail.com');

# prints 0
print is_single('hpotter@hotmail.com, mickymouse@disney.com');
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But I guess some people around here have a far more beautiful solution.

Update strict-ing the code and making it more verbose