That way it won't work:

$ perl -MProc::ProcessTable -le 'print for @{Proc::ProcessTable->new->
+table}'
Proc::ProcessTable::Process=HASH(0x814cb3c)
Proc::ProcessTable::Process=HASH(0x827d44c)
Proc::ProcessTable::Process=HASH(0x8276e84)
Proc::ProcessTable::Process=HASH(0x827d590)
Proc::ProcessTable::Process=HASH(0x8276f74)
Proc::ProcessTable::Process=HASH(0x8277890)
Proc::ProcessTable::Process=HASH(0x8277a40)
Proc::ProcessTable::Process=HASH(0x8277bf0)
...
[download]

Your match has to become

$_->pid =~ /^$wanted$/
[download]

But even then it would be an abuse of grep and of a pattern match, IMHO. Both slightly suboptimal, for

• the implicit grep cycle will continue also after the pid has been matched;
• the match is either less efficient than a comparison or optimized to a comparison, I can't remember which is which. In any case why not going for an explicit comparison to begin with?