unless( $a ne $b ) {

##</code><code>##

if( $a eq $b ) {

# or

if( $a ne $b ) {

##</code><code>##

    croak( "You must pass foo() a non-empty array ref" )
        unless  $arg  &&  'ARRAY' eq ref($arg)  &&  0 < @$arg;

##</code><code>##

    croak( "You must pass foo() a non-empty array ref" )
        if  ! $arg  ||  'ARRAY' ne ref($arg)  ||  0 == @$arg;

##</code><code>##

    croak( "You must pass foo() a non-empty array ref or hash ref" )
        unless  'ARRAY' eq ref($arg)  &&  0 < @$arg
            ||  'HASH'  eq ref($arg)  &&  0 < keys %$arg;

##</code><code>##

    croak( "You must pass foo() a non-empty array ref or hash ref" )
        if(     (  'ARRAY' ne ref($arg)  ||  0 == @$arg  )
            &&  (  'HASH'  ne ref($arg)  ||  0 == keys %$arg  )
        );

##</code><code>##

    croak( "You didn't pass foo() a reference" )
        if  ! $arg  ||  ! ref $arg;
    if(  'ARRAY' eq ref $arg  ) {
        croak( "You passed foo() an ARRAY ref but it was empty" )
            if  0 == @$arg;
        ...
    } elsif(  'HASH' eq ref $arg  ) {
        croak( "You passed foo() a HASH ref but it was empty" )
            if  0 == keys %$arg;
        ...
    } else {
        croak(
            "You passed foo() a ", ref($arg),
            " ref (must be ARRAY or HASH)",
        );
    }