in reply to ticked off with backticks

my $output = `gnuplot $cmd 2>&1`;

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That will merge the STDERR output of gnuplot into STDOUT, so it'll all end up together in $output. Then you go from there in terms of working out your issues.

BTW, you might have better luck using IPC::Run, if you need to separate STDOUT and STDERR: 

use IPC::Run qw( run );
my ($stdout, $stderr);
run ["gnuplot", $cmd], undef, \$stdout, \$stderr;

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Something like that. 
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:Wq
Not an editor command: Wq

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In Section Seekers of Perl Wisdom