Autovivification is when perl creates records in a hash or array after dereferencing an undef. What you are describing is simply that an undef in string context is rendered as the empty string, just like an undef in numeric context is 0 and an undef in logical context is false.

When turning off warnings, you should turn off the minimal set for your target scenario, e.g.

{
  no warnings 'uninitialized'; # remove to see what happens
  print $a + $b;
}
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Usually it means you have a typo in a variable,

Except the OP said they have strict enabled, so in that scenario they would get a fatal error, not a warning. Which is why I asked the OP for a larger block of code to inform context.