$ perl -Mstrict -MDBI -E 'my$dbh=DBI->connect("dbi:Pg:");

$dbh->{pg_bool_tf} = 0; #  1/0
print $dbh->selectall_arrayref("select 1=1")->[0]->[0], "\n";

$dbh->{pg_bool_tf} = 1;  #  t/f 
print $dbh->selectall_arrayref("select 1=1")->[0]->[0], "\n"; # 
'

output: 

1
t