A cleaned up copy of what you posted:

#!/usr/bin/perl

use strict;
use warnings;

my @cmd = ('/sbin/route');
push @cmd, 'add'; 
push @cmd, 'default'; 
push @cmd, 'gw',
push @cmd, '172.16.0.200', 
push @cmd, 'dev'; 
push @cmd, 'enxb827eb95400d'; 
system(@cmd);
[download]

some lines end with commas when they should be semicolons:

#!/usr/bin/perl

use strict;
use warnings;

my @cmd = ('/sbin/route'); 
push @cmd, 'add'; 
push @cmd, 'default'; 
push @cmd, 'gw'; 
push @cmd, '172.16.0.200'; 
push @cmd, 'dev'; 
push @cmd, 'enxb827eb95400d'; 
system(@cmd);
[download]

If in doubt, print what your variables contain. Tutorials->PerlMonks for the Absolute Beginner/Basic debugging checklist.

Also:

my @cmd = qw[/sbin/route add default gw 172.16.0.200 dev enxb827eb9540
+0d];
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or

my @cmd = split/\ +/, "/sbin/route add default gw 172.16.0.200 dev enx
+b827eb95400d";
[download]

perl -e 'use MIME::Base64; print decode_base64("4pmsIE5ldmVyIGdvbm5hIGdpdmUgeW91IHVwCiAgTmV2ZXIgZ29ubmEgbGV0IHlvdSBkb3duLi4uIOKZqwo=");'

You haven't shown any error messages, so we can't say what's going wrong. (Also, please use <code> tags to format code.) It looks like you're not checking the return value of system for errors, you should do that with the code shown in its documentation, or with a module such as IPC::System::Simple. I wrote about the topic of running external commands at length, with sample code, here.