If I understand you correctly, you want us to write code calling the function that would compile. The following not only compiles, but also runs without problems (in 5.26.1):

#!/usr/bin/perl
use warnings;
use strict;

sub mixedregex {
    s[\\0([0-7]{3})][chr oct $1]egr;
}

$_ = '\0123';
print mixedregex();  # S
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