`your_command | tee test.txt` =~ /your_capturing_regex/

##</code><code>##

$ perl -le '`date | tee test.txt` =~ /^(\w+)/; print $1'
Tue

##</code><code>##

$ cat test.txt
Tue 18 Mar 2014 11:02:34 EST

##</code><code>##

system "echo @{[`your_command` =~ /your_capturing_regex/]} > test.txt"

##</code><code>##

$ perl -e 'system "echo @{[`date` =~ /^(\w+)/]} > test.txt"'

##</code><code>##

$ cat test.txt
Tue