is_a doesn't check if the object is a descendant of the specified class.

sub is_a {
    croak "is_a() expects a package name"
        if (scalar @_ != 1 || !defined $_[0] || ref $_[0] ne '');

    my $type = shift;
    return sub {
        return 0 if (!defined $_[0]);
        return (ref $_[0] eq $type) ? 1:0;
    };
}
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So use is_a('mSomeObject_T') or make your own constraint.

use Scalar::Util qw( blessed );

sub is_or_extends {
    my ($type) = @_;

    croak "is_or_extends() expects a package name"
        if !defined($type) || ref($type) ne '';

    return sub {
        return ( blessed($_[0])
           ? $_[0]->isa($type)
           : UNIVERSAL::isa($_[0], $type)
        );
    };
}
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