This is the only line that could generate that message (although it doesn't look like line 35):

my $monit_percentual = (  $avail  /  $avail+$used  ) * 100 ;
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UPDATE (dave_the_m): That means $avail is probably 0 ($avail+$used)=0 (probably both variables are 0). Tip #2 from the Basic debugging checklist: print

Also:

• How do I compose an effective node title?
• Filesys::DiskSpace looks like it may have been abandoned. It has several open issues, and the last update was more than 17 years ago.