If by "multiline text" you mean feed it input via STDIN, you can explicitly open a pipe using open, e.g.
in reply to $var | system()
open my $stream, "| mail -s '123' email\@at.com" or die "pipe fail: $!
print $stream $text; # Or @lines, or $line1, $line2...
close $stream; # Handle needs to close or go out of scope to clear buf
+fer and send EOF
There are a large number of other possibilities here, depending on how complex you want to get: system; pipe, fork and exec; IPC::Open2; IPC::Open3... See perlipc for some gory detail.
#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.