One way:

use File::Slurp "write_file";
write_file("filename", $mac) or die "Error writing to file: $!";
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or, more traditionally,

open my $handle, "> filename" or die "Couldn't open file: $!";
print $handle $mac;
close $handle;
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(Ideally, you should check if the print and close fail also; this is commonly overlooked.)