I just want to check if this is a viable way to clear or initialize a set of hashes:

No. Your code does nothing.

This bit: %_ = () would assign nothing to the hash %_; which is probably empty anyway.

But that code is never invoked because this bit: for  my(%hash1,%hash2,%hash3); doesn't result in anything, so the for never iterates.

The overall effect of you code is exactly the same as just: my(%hash1,%hash2,%hash3);

And the only reason "it seems to work fine"; is because it is not necessary to clear or initialise variables when you declare them.

Any variable declared with my, is a new clean, empty variable. You don't need to do anything to it to make it so.

"my" variable %hash1 masks earlier declaration in same scope
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So, the statement is equivalent to

my (%hash1, %hash2, %hash3);
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Note that the hash %_ is not cleared, because the body of the loop is not run (the brand new hashes are emtpy).

Without my, %_ would be cleared, but %hash1 and company would stay untouched. To test, just play with comments in the following code:

# use strict;
use warnings;
%_     = (u => 2);
%hash1 = (b => 3);
# %hash1 = (b => 3);
# %_ = () for my (%hash1,%hash2,%hash3);
# %_ = () for (%hash1,%hash2,%hash3);
print %_, %hash1, "\n";
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To clear a hash, use undef or %hash = ().

Thanks! both replies helped a lot. I thought it looked a little funky and thought I would check so I am really glad I did. Any suggestion on the best way to clear all of them? At initialization I can just do:

my(%hash1,%hash2,%hash3);

but later on if I want to clear all of them how should I go about it? I mean I could just clear each one individually, but when I have a long list that can be a bit of a pain...

but later on if I want to clear all of them how should I go about it?

The simplest way is to scope them such that they get reinitialised by re-entering the scope each time you want to re-use them:

for( some loop condition or contruct ) {
    my( %hash1, %hash2, %hash3 );
    ## do stuff with them ...
}
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Each time that loop iterates, the hashes will be reinitialised for you.

But, it you really need to do it manually, you could use:

undef %$_ for \(%h1,%h2,%h3);
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You can do

( %hash1, %hash2,  %hash3 ) = ();
# or
%hash1 =  %hash2 = %hash3   = ();
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لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

If you have a bunch of hashes (could also be a bunch of arrays) that you need to define en masse and identically initialize as non-empty, something like this works:

>perl -wMstrict -MData::Dump -le
"use constant HASH => qw(foo bar  1 one);
 ;;
 %$_ = HASH for \my (%hash1, %hash2);
 dd \%hash1;
 dd \%hash2;
"
{ 1 => "one", foo => "bar" }

{ 1 => "one", foo => "bar" }
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If you really have a long list of hashes, you could store them as an array or hash of hash references. In which case you can clear them using a loop or even clear the array or hash that contains them.