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

##</code><code>##

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

##</code><code>##

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