In snippet 1:

$long_content = ...   # Long string copy 1.
$new_contents = ...   # Long string copy 2.
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In snippet 2:

$main_vars{content} = ...   # Long string copy 1.
$old_content = ...          # Long string copy 2.
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They are going to be similar since the parts you said are slow occur in both versions. Moving the slow parts around doesn't make things faster.