This works for me:

$_ = q{D3E' C@$o*r(-A[].Mick7};

s/[^A-Za-z\d\s\-.\']//g;

print "$_\n";
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I suspect you're quoting your test string with double quotes, and the $o is interpolating into the string - it's most likely undefined, and thus never shows up in your output.

$,=42;for(34,0,-3,9,-11,11,-17,7,-5){$*.=pack'c'=>$,+=$_}for(reverse s
+plit//=>$*
){$%++?$ %%2?push@C,$_,$":push@c,$_,$":(push@C,$_,$")&&push@c,$"}$C[$#
+C]=$/;($#C
>$#c)?($ c=\@C)&&($ C=\@c):($ c=\@c)&&($C=\@C);$%=$|;for(@$c){print$_^
+$$C[$%++]}
[download]