use strict;
use warnings;

my %hash;
while (<DATA>) {
    chomp;
    my ($dept,$email)=split(/:/);
    push @{ $hash{$dept} }, $email;
}

for (sort keys %hash) {
    print "$_:", join(',', @{ $hash{$_} }), "\n";
}

__DATA__
sales:bob@foo.com
sales:joe@foo.com
retail:steve@bar.com
sales:debbie@foo.com
sales:john@foo.com
support:david@blah.com
retail:judy@bar.com
support:jose@blah.com

##</code><code>##

retail:steve@bar.com,judy@bar.com
sales:bob@foo.com,joe@foo.com,debbie@foo.com,john@foo.com
support:david@blah.com,jose@blah.com