Bonus points if you actually explain why

Probably obvious to most monks

Here goes. It is a case of modifying an array in a for loop. Before the for loop, the @searchTexts array contains 5 elements (indices: 0-4). The 1st 5 times though the loop, the array size remains constant at 5. Now, let's look at the end condition of the loop:

$i <= @searchTexts +1 -1;
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which is just a slightly obfuscated form of:

$i <= @searchTexts;
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Since the array is evaluated in scalar context, @searchTexts is the same as scalar @searchTexts, which returns the current size of the array. The 1st 5 times through the loop, the size is 5, and therefore the end condition is satisfied (the 5th time , $i is 4, which is less than or equal to 5). At the end of the 5th time through the loop, $i increments to 5, which is still less than or equal to 5. This causes the loop to be executed a 5th 6th time, which in turn causes the array to grow by 1 because of this line:

  @searchTexts[$i] =~ s/^\s+//;
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which resolves to this:

  @searchTexts[5] =~ s/^\s+//;
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From there on, the array continues to grow by one each time through the loop because the end condition is dependent on the array size.