Try running¹ Run your $backupcommand in a subshell and redirect the output in its entirety:

...
my $result = `( $backupcommand ) 2>&1`;
[download]

P.S.: you can interpolate variables into a double-quoted string, which makes it somewhat easier to read:

my $backupcommand = "tar -cvf - $directory/$seasonYear*.html | 7z a -m
+x9 -si $backupdirectory/$seasonYear.tar.7z";
[download]

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¹ upd: just tried it myself — works fine.