http://qs1969.pair.com?node_id=1177286


in reply to Re^4: Looking for suitable spells to get open to return a filehandle from a module
in thread SOLVED: Looking for suitable spells to get open to return a filehandle from a module

... so Symbol::gensym is technically no longer needed

But there are times when it's still useful... like when you want open3() to keep STDOUT and STDERR separate (by default, if you pass it undef for both STDOUT and STDERR, STDERR will be directed to the same filehandle that's created for STDOUT)

#!/usr/bin/env perl use warnings; use strict; use IPC::Open3; use Symbol qw(gensym); use POSIX qw(:sys_wait_h); # I'm sometimes using perl 5.8.5, so //'<undef>' unavailable # verified later on 5.14.2 sub defor($) { defined($_[0]) ? $_[0] : '<undef>' } foreach my $run (1..3) { my ($out, $err) = ('',''); my ($fhi, $fho, $fhe) = ($run == 1) ? (undef, undef, undef) : # when fhe==un +def goes to open3, STDERR will be redirected to STDOUT ($run == 2) ? (undef, undef, gensym) : # this keeps S +TDERR and STDOUT separate (gensym, gensym, gensym) ; # as does this +, the first two gensym are unecessary, but the third is needed my $pid = open3( $fhi, $fho, $fhe, qq[cat - ; perl -le 'print STDE +RR "this should be STDERR (run#$run)...\nand so should this (run#$run +)\n"'] ); print { $fhi } "[run#$run] This should be STDOUT\n"; close($fhi); $out .= join '', map { s/^/STDOUT> / if $_ ne $/; $_ } readline($f +ho) if defined $fho; $err .= join '', map { s/^/STDOUT> / if $_ ne $/; $_ } readline($f +he) if defined $fhe; print "Read STDOUT:\n$out\n" if length($out); print "Read STDERR:\n$err\n" if length($err); printf "run#%d --- fhi='%s' fho='%s' fhe='%s'\n", $run, defor $fhi +, defor $fho, defor $fhe; print "="x75, "\n\n"; } __END__ Read STDOUT: STDOUT> [run#1] This should be STDOUT STDOUT> this should be STDERR (run#1)... STDOUT> and so should this (run#1) run#1 --- fhi='GLOB(0x92fdba4)' fho='GLOB(0x92fdbcc)' fhe='<undef>' ====================================================================== +===== Read STDOUT: STDOUT> [run#2] This should be STDOUT Read STDERR: STDOUT> this should be STDERR (run#2)... STDOUT> and so should this (run#2) run#2 --- fhi='GLOB(0x93811c0)' fho='GLOB(0x932bc60)' fhe='GLOB(0x92fd +ba4)' ====================================================================== +===== Read STDOUT: STDOUT> [run#3] This should be STDOUT Read STDERR: STDOUT> this should be STDERR (run#3)... STDOUT> and so should this (run#3) run#3 --- fhi='GLOB(0x93811c0)' fho='GLOB(0x932bc60)' fhe='GLOB(0x92fd +ba4)' ====================================================================== +=====

Yes, I know the OP didn't include open3, but since I was recently learning the ins and outs of open3 filehandles, I thought I'd mention this exception to your general rule, for posterity's sake.