Not exactly. By using my, you declare new hashes. If you used warnings, Perl would tell you:

"my" variable %hash1 masks earlier declaration in same scope
[download]

So, the statement is equivalent to

my (%hash1, %hash2, %hash3);
[download]

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";
[download]

To clear a hash, use undef or %hash = ().

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