sub A {
    my ($OBJ,$p) = @_;
    if ( exists $OBJ->{$p} && exists $OBJ->{'sub'}) {
        my $q = $OBJ->{$p};
        $q = $OBJ->{'sub'}->($q); # execute the sub stored in $OBJ
        print STDOUT "$p=$q\n";
    }

    my $formatThis = sub {
      my ($x) = @_;
        my $y = sprintf ( "%.2f", $x);
        return $y;
    };
    $OBJ->{'sub'} = $formatThis;
}
# test it
my $ref = {};
$ref->{'float'} = 3.1415926;
A($ref);         # get a sub
A($ref,'float'); # now doit