$outputfile is always empty ... thats not good ... and you're not checking to see that open failed ( autodie can check for you)

Try this, it comes with error checking

#!/usr/bin/perl --
use strict; use warnings;
use Path::Tiny qw/ path /;
for my $infile ( glob "*.dat" ){
     my $outfile = "$file.bak" ;
     path( $infile )->copy( $outfile );
}
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