Well, there's the easy (but probably risky) way and the harder (but safer) way to do it. The easy way is just string eval it:

my $strFraction = eval "1/2";

print "strFraction is actually $strFraction <br>";
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but consider:

my $strFraction = eval qq|print "this could have been `rm *` - bye bye
+ files!\n"|;

print "strFraction is actually $strFraction <br>";
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which prints:

this could have been `rm *` - bye bye files!
strFraction is actually 1 <br>
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The harder way is to parse the expression, or at the very least untaint it using an appropriate regex if all you accept are simple fractions"