my $h_b = {};
my $x   = eval {
    use warnings FATAL => 'uninitialized' ;
    $h_b->{velocity}." mph";
} // 'unknown';

print $x;

##</code><code>##

sub catch_undef (&) {
    my $code_ref = shift;
    
    eval {
        use warnings FATAL => 'uninitialized' ;
        $code_ref->()
    };
}