What's going wrong in your code is that $1 contains the string "0xdf7780", which Perl does not automatically convert to a number. You can use the hex function to convert it to a number first, and then use sprintf to convert the output back to hex:

$line1 =~ /addr:([a-z0-9-]+)\s+/;
my $one = $num + hex $1;
$one = sprintf "0x%x", $one;
$line1 =~ s/addr\:0x.*qospri/addr\:$one qospri/;
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However, it's also possible to do it all in one statement, using the special s///e, which runs the replacement part as Perl code and inserts its return value as the replacement, so you can use it to run sprintf directly. I've also used \K and a (?=...) lookahead to exclude those strings from the match, so they're not changed by the replacement.

$line1 =~ s{ addr:0x \K ([0-9a-fA-F]*) (?=\s+qospri) }
    { sprintf "%x", hex($1)+$num }xe;
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By the way, you should always Use strict and warnings, and use proper indentation to make your code easier to read (perltidy can help with that).

Update: I added the /x modifier to the regex to make it easier to read. Also switched from (?=\ qospri) to (?=\s+qospri) to make it equivalent to the original regex.