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