use File::Basename        qw( basename );
use File::Spec::Functions qw( catfile rel2abs );

chomp(my $filenameful = <STDIN>);
$filenameful  = rel2abs($filenameful);
my $filename  = basename($filenameful);
my $backupdir = 'backup_files';
my $back      = $filename . ".bak";
my $backful   = rel2abs(catfile($backupdir, $back));