You don't normally need to initialize (autovivification will take care of this) but you can use the ||= construction:

for (@somelist) {
      my($foo,$bar) = split;
      my $x = somecalc($foo,$bar);
      $somehash{ $foo }->{ $bar } ||= [];
      push @{ $somehash{ $foo }->{ $bar } }, $x;
}
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||= basically says 'if the left-hand value isn't defined, assign the right-hand side to it'.

Chris
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