Just do
$words =~ s/\s\s+$/ /;

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whithout any if. If there is multiple whitespace, it will be replaced, if not, nothing happens.
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: Replacing whitespaces with a regex by choroba
in thread Replacing whitespaces with a regex by Dr Manhattan

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