Aye, /r (introduced in 5.14) is key here.

You can use /r on the s/// too. 

my $res = $str =~ s{ (\d+) }{ $1 =~ s/(\d)/3/gr }xer;

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And we don't need those captures. 
my $res = $str =~ s{ \d+ }{ $& =~ s/\d/3/gr }xer;

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In reply to Re^2: regex in REPLACEMENT in s/// by ikegami
in thread regex in REPLACEMENT in s/// by perlboy_emeritus

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