my $infile = $ARGV[0];
infile ($infile);

sub infile {

open( my $infile, '<', $ARGV[0] ) or die "cannot open file: $!";
# ...

##</code><code>##

my $infile = $ARGV[0];
process_file ($infile);

sub process_file {
     my $current_file = shift; # or: my $current_file = $_[0];
     open my $FILEHANDLE, '<', $current_file or die "cannot open file $current_file: $!";
     # ...