I think it would be helpful, if you slightly change your data structure, or add a new one:

%rs = (
    vdd  => 'P',
    vdda => 'P',
    vssa => 'G',
    vss  => 'G',
);
[download]

Then use this table to modify your input $line.

$line = "*.PIN foo:I bar:O vdd:I vssq:I vssa:O";
$line =~ s/\b(\w+):([A-Z])\b/$rs{$1} ? "$1:$rs{$1}" : "$1:$2"/eg;
[download]

/prakash