Another statement of the problem might be "after 'addr:0x', replace 1 or more hex digits (or maybe just a single '0'?) with a string from a variable". The s/// match pattern might be expressed as
    (?<= addr:0x)  # after a certain literal string
    [[:xdigit:]]+  # 1 or more hex digits
(Update: See "(?<=pattern)" (and friends) in Extended Patterns, "Lookaround Assertions" subsection, in perlre; Looking ahead and looking behind in perlretut. See also POSIX Character Classes in perlrecharclass for [[:xdigit:]] and similar.)

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = qq{0 (0) tracegen chn:req cmd:SDP_CMD_RDSIZED unit:0x10 addr:
+0x0 len:0x3f vc:0 qospri:0 qosfw:0 strm:0x0 chain:0 io:0\n};
 ;;
 my $address = '500000000';
 ;;
 $s =~ s{ (?<= addr:0x) [[:xdigit:]]+ }{$address}xms;
 ;;
 my $expected = qq{0 (0) tracegen chn:req cmd:SDP_CMD_RDSIZED unit:0x1
+0 addr:0x500000000 len:0x3f vc:0 qospri:0 qosfw:0 strm:0x0 chain:0 io
+:0\n};
 print 's/// ok' if $s eq $expected;
"
s/// ok
[download]

If you have Perl version 5.10+, you can use the more flexible  \K (see Extended Patterns) positive look-behind assertion operator:
    $s =~ s{ addr:0x \K [[:xdigit:]]+ }{$address}xms;
This allows the positive look-behind pattern to be held in a variable and managed separately:
    my $tag = qr{ addr:0x }xms;
    $s =~ s{ $tag \K [[:xdigit:]]+ }{$address}xms;
(Update: The advantage of such separate management is that a more complex pattern may be built up:
     my $tag = qr{ (?: addr | foo | bar) :0x }xms;)

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