perl -ple "BEGIN{open $to,'>',shift @ARGV;select $to}" destination.
+txt source1.txt source2.txt
[download]

The -p print each line, -l does automatic lines handling; the BEGIN block shift @ARGV using that file as destination, select print everything to the destination.

PS: if you want something that can print to a destination file or to STDOUT you can modify the above in:

perl -ple "BEGIN{open $to,'>',shift @ARGV and select $to if $ARGV[0]}"

# to STDOUT
perl -ple "BEGIN{open $to,'>',shift @ARGV and select $to if $ARGV[0]}"
+ 0 src1 src2

# to file
perl -ple "BEGIN{open $to,'>',shift @ARGV and select $to if $ARGV[0]}"
+ dst src1 src2
[download]

PPS: maybe this is more intellegible

perl -ple "BEGIN{open $to,'>',shift @ARGV and select $to unless $ARGV[
+0] eq 'STDOUT'}"

# to STDOUT
perl -ple "BEGIN{open $to,'>',shift @ARGV and select $to unless $ARGV[
+0] eq 'STDOUT'}" STDOUT src1 src2
[download]

L*