The trick is to split the numbers already when capturing: 
$line =~ s/(\d\d)(\d\d)/$1 $2/g;

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Update: Note that /d and \d are different.

لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: substituting capture groups substrings by choroba
in thread substituting capture groups substrings by naturalsciences

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