use Data::Dumper;
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BEGIN {
   require Data::Dumper;
   Data::Dumper::->import();
}
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Like psini already said, the problem is that Perl already had to decide whether Dumper is a function call or not before use Dumper; was encountered.

1. Compile my %test_hash = ( a => { b=> 'c' } );
2. Compile print Dumper %test_hash; (Dumper is treated as a *Dumper since no function named Dumper was found)
3. Compile use Data::Dumper;
4. Execute require Data::Dumper;
5. Execute Data::Dumper::->import();
6. [ Script has been compiled. Now start executing it ]
7. Execute my %test_hash = ( a => { b=> 'c' } );
8. Execute print(*Dumper %test_hash);

You can only omit parens on a function call to a function that's already been declared (but not necessarily defined) or if you annotate the function call with &.

Update: Added list showing execution flow.

Thanks all... it now makes sense...

I don't think it is exactly so. When execution reach the print line, Data::Dumper is available. If you try the code from my previous example (I tested it, at last) you'll see that it works.

The problem is that at compile time, the Dumper sub is not imported till use Data::Dumper is reached; so "print Dumper something", without parenthesis, is interpreted as if Dumper were a filehandle to print to.

That's true (and something I didn't know psini++), but I think the op was expecting

use Module;
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to be equivalent with

BEGIN{require Module;}
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, which it is not.

Sorry but, in this example at least, it is equivalent.

Both use and BEGIN are executed at compile time, but only when the compiler reach their position. So:

use strict;
use warnings;

my %test_hash = ( a => { b=> 'c' } );
print Dumper %test_hash;

BEGIN{require Data::Dumper; Data::Dumper->import}
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gives the same error as above where

use strict;
use warnings;

my %test_hash = ( a => { b=> 'c' } );
print Dumper(%test_hash);

BEGIN{require Data::Dumper; Data::Dumper->import}
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works, for exactly the same reason.

Rule One: "Do not act incautiously when confronting a little bald wrinkly smiling man."