-M is just a weirdly named function, really.

if (-M $outputfile < $timeforfiletoupdate_beforeerror) {
   # etc
}
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is the same as

if (-M($outputfile) < $timeforfiletoupdate_beforeerror) {
   # etc
}
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And like every other function, nothing's stopping you from assigning its result to a variable.

$days_since = -M $outputfile;
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