Since you only use the parameter once, it's cheaper to just reference it as $ARGV[0] pop it inline rather than assign it to anything.

@b=(0,1);push@b,$b[-2]+$b[-1]for 2..pop;print$b[-2]
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You can save fourteen more strokes (and, irrelevantly, memory) by not using an array and using convoluted scalar assignment with a variable that is pre-initialized:

$b=1;$?=($b+=$?)-$?for 2..pop;print$?
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