Don't. 

GetOptions(
    '--one'   => \$one,
    '--two'   => \$two,
    '--three' => \$three,
);

die "Only one of options --one, --two and --three are allowed\n"
   if ( grep defined, $one, $two, $three ) > 1;

[download]
or 
GetOptions(
    '--one'   => \$opts{one},
    '--two'   => \$opts{two},
    '--three' => \$opts{tree},
);

die "Only one of options @{[ sort keys %opts ]} are allowed\n"
   if ( grep defined, values(%opts) ) > 1;

[download]

Update: Updated since I hadn't properly read the question initially.

In reply to Re: How can I turn a string into a variable name? by ikegami
in thread How can I turn a string into a variable name? by mnooning

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