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sub map_split {
    my $ofs = 0;
    join "", map { $ofs += length; $_ => substr $s2, $ofs++, 1 } split
+ /\0/, $s1, -1;
};

sub map_split_join {

    my $ofs = 0;
    join "", map { $ofs += length; $_ . substr $s2, $ofs++, 1 } split 
+/\0/, $s1, -1;
};

                 Rate map_split_join          subst      map_split
map_split_join 1697/s             --            -7%           -13%
subst          1828/s             8%             --            -6%
map_split      1949/s            15%             7%             --