Another way would be to use substr to insert spaces working back from the right-hand end of the string.

$ perl -E '
> $str = q{abcdefghijklmn};
> say $str;
> substr $str, $_, 0, q{ } for
>    grep { ! ( $_ % 4 ) } reverse 4 .. length( $str ) - 1;
> say $str;'
abcdefghijklmn
abcd efgh ijkl mn
$
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I hope this is of interest.