PHP, 84 79 70 63 + 1 bytes

for(;$c=$argn[$i++];$a+=$c==";")$c^A^R||$a=~-print chr($a%127); 

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PHP, 84 79 70 1 + 63 = 64 bytes

for(;$c=$argn[$i++];$a+=$c==";")$c^A^R||$a=~-print chr($a%127); 

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[PHP], 84 79 70 63+1 bytes

for(;$c=$argn[$i++];$a+=$c==";")$c^A^R||$a=~-print chr($a%127); 

Run as pipe with -nR

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PHP, 84 79 70 63 + 1 bytes

for(;$c=$argn[$i++];$a+=$c==";")$c^A^R||$a=~-print chr($a%127); 

Run as pipe with -nR

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PHP, 84 79 70 bytes

<?for(;$i=fgetc(STDIN);$c+=$i==';')if($i=='#'&&print chr($c%127))$c=0; 

Reads from stdin.

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From 84 to 79. I replaced the charcodes with the strings to remove unnecessary ord function. Replaced nested braces with conditional &&.

From 79 to 71. I removed the initialization of $c. This generates a PHP notice which can be suppressed via the ini (you can also just execute php -d display_error=0. Also switched to short tags. See Code Golf PHP Notices.

[PHP], 84 79 70 63+1 bytes

for(;$c=$argn[$i++];$a+=$c==";")$c^A^R||$a=~-print chr($a%127); 

Run as pipe with -nR

Try it online!