use Encode;
use DBI;  # (or whatever you use for your PostgreSQL)

my $dbstr; # suppose this holds "unicode" data from the DB

# ... do whatever it takes to fetch a value into $dbstr;
# since DBI might not be "unicode-aware", you may need to
# coerce perl into treating the value as unicode:

my $unistr = decode( 'utf8', $dbstr );
my $latin1str = encode( 'iso-8859-1', $unistr );

print $latin1str;