The immediate problem is that $rec_type and $y_label need to be global to the if/elsif code. If you have access to Perl 5.10.x however you can use given/when to clean up the code somewhat. Consider:

use strict;
use warnings;
use 5.010;

my $rec_type;
my $y_label;

given (param('P1P2_bugs')) {
    when ('P1') {
        $rec_type = '3';
        $y_label  = 'Open P1 Bugs';
    }
    when ('P2') {
        $rec_type = '4';
        $y_label  = 'Open P2 Bugs';
    }
    when ('P1-P2') {
        $rec_type = '2';
        $y_label  = 'Open P1-P2 Bugs';
    }
    when ('Daily-Snapshot') {
        $rec_type = '1';
        $y_label  = 'Daily-Snapshot';
    }
    default {
        die "Unrecognised parameter type: $_\n";
    }
}
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Note that the two variables that are being set are global to the given block and that there is a default handler for unexpected results from param.