To convert a fraction to decimal equivalent (or approximation, if the fraction doesn't divide out cleanly):

if ($fraction =~ /^((-) ?)?((\d+) )?(\d+) ?\/ ?(\d+)$/) {
    $fraction = ($2 ? -1 : 1) * ($4 + $5 / $6); 
}
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To test for a negative:

if ($fraction =~ /^((-) ?)?((\d+) )?(\d+) ?\/ ?(\d+)$/ && $2) {
    #run if negative...
}
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