Use use warnings;! (And use use strict; if you're not) "1/2" is not a valid number.

>perl -wle"print '1/2'+0"
Argument "1/2" isn't numeric in addition (+) at -e line 1.
1

>perl -wle"print '1abc'+0"
Argument "1abc" isn't numeric in addition (+) at -e line 1.
1

>perl -wle"print '123/456'+0"
Argument "123/456" isn't numeric in addition (+) at -e line 1.
123
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The simplest safe solution is probably to use Math::BigRat.

>perl -MMath::BigRat -wle"print Math::BigRat->new('1/2')->numify();"
0.5
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Math::BigRat is part of Perl.