An edit in place with deliberate assumptions on the source file (actually, I get this from dwm042's example):

$ cat target-file.txt
Address1=
Address2=
Address3=

$ perl -Mstrict -pi -wle 'our @ports; BEGIN { my $source = "tcp 20 tcp
+ 40 tcp 80"; @ports = $source =~ /tcp\s+(\d+)/g;} $_ .= shift(@ports)
+;'  target-file.txt

$ cat target-file.txt
Address1=20
Address2=40
Address3=80
[download]

(johngg demonstrates the verbose version of -pi) Or, if you want to keep the original file and redirect the result to other file, you can use -n instead of -p and -i combination. Well, -i.ext keeps the original version with .ext appended to the filename.

$ cat target-file.txt
Address1=
Address2=
Address3=

$ perl -Mstrict -wnle 'our @ports; BEGIN { my $source = "tcp 20 tcp 40
+ tcp 80"; @ports = $source =~ /tcp\s+(\d+)/g;} $_ .= shift(@ports); p
+rint' target-file.txt  > output.txt

$ cat output.txt
Address1=20
Address2=40
Address3=80

$ cat target-file.txt
Address1=
Address2=
Address3=
[download]

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