Very simply, you are doing an assignment rather than a test for equality.

Change

if ( $status = $on_commercial ) 
{
    print "The ups is normal\n";
} 
if ( my $status = $on_battery ) 
{ 
    print "The ups is on battery\n";
}
[download]

to:

if ( $status == $on_commercial ) 
{
    print "The ups is normal\n";
} 
if ( my $status == $on_battery ) 
{ 
    print "The ups is on battery\n";
}
[download]

... and you should be all set.

If you need to compare strings instead of values, use eq instead of ==.