At the end of the while loop's body, say $prev_line= $_; to save what you have before doing <I> again.

I'd also go ahead and used a named variable instead of relying on $_ to still hold the line. Something like:

my ($current_line, $prev_line);
while($current_line=<I>){
    chomp;
    @line = split(/\t/, $_);
    if($line[0] eq $GET{'id'}){
        $found=1;
        last;
    }
    $prev_line= $current_line;
}
close(I);
# value in $prev_line still good.
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