You are using the /m option but you are not using either of the ^ or $ anchors.   It looks like you should be using the /s option so that . will match a newline as well as other characters.

$s.=$_ while <STDIN>;$s=~s/.../.../;
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... lacks slurp mode and introduces an unneeded variable $s:

undef$/;$_=<STDIN>;s/.../.../;
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$#ARGV<0&&exit 1;
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Much typing to test if @ARGV has elements, and exit 1 can be shortened to die, if the additional error message does not disturb:

@ARGV||die;
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perl -e'use re "eval";';$#ARGV<0&&exit 1;...'
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... can be shortened on the perl command line:

perl -Mre=eval -e '@ARGV||die;...'
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Alexander