Here's another way, using the Tie::File module:

#!/usr/bin/perl

use strict;
use warnings;
use Tie::File;

my $mac = "this is the variable to be written to a file";

# tie array to output file
tie my @array, 'Tie::File', "outputfile.txt" or die "can't open file: 
+$!";

# write $mac to file
push @array, $mac;

# all finished
untie @array;

__END__
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