my %C = (%A, %B);
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If you were dealing with hash references, you would do:

my $C = { %$A, %$B };
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The PDSC is an excellent resource for learning how to grok and manipulate Perl data structures.

Update: As pointed out below by muba and CountZero, you need to keep in mind that hash keys must be unique, and as such any keys in hash A that also exist in hash B will get clobbered.

Cheers,
Darren