If you want to obtain the entire expression, not only the fraction, you can try this:

while (<DATA>) {
    s!\d+/\d+!$&!gee;
    print;
}
__DATA__
SYMM    1/8 - X, 1/8 + Y, 1/8 + Z
SYMM    1/6 - X, 1/6 + Y, 1/6 + Z
SYMM    1/4 - X, 1/4 + Y, 1/4 + Z
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which basically matches all the fractions and substitute them by themselves evaluated (The numeric value)

Hopes

$_=$,=q,\,@4O,,s,^$,$\,,s,s,^,b9,s,
$_^=q,$\^-]!,,print
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