# I am looking for hosts that appear in all of a set of SQL tables. 
# I keep a table count and increment the count for a host for each table in 
# which it occurs.
# Then I clean out those that were not in all tables.

# Faked data
my %hosts = (
    a => 3,
    b => 3,
    c => 4,
    d => 3,
    e => 2,
    f => 3,
    g => 3,
);
my $table_count = 3;

# This:
do {delete $hosts{$_} unless $hosts{$_} == $table_count} for keys %hosts;

# or This:
map {delete $hosts{$_} unless $hosts{$_} == $table_count} keys %hosts;