this bit:

my $gpg_command = `gpg --trust-model always -er 12345678 my_file.txt`;

print defined($gpg_command) ? $gpg_command : "Command failed\n"
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But in your open command, you're effectively trying to open a pipe to a command that matches the output from your original backtick command, which should be failing I'd think.

remember that backticks do execute your command, and things get a little simpler. For example:

use strict;
use FileHandle;

my $fh = FileHandle->new('capture.out',"w");

print $fh `dir`;
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