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

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

my $infile = $ARGV[0];
process_file ($infile);
...
     my $current_file = shift; # or: my $current_file = $_[0];
     open my $FILEHANDLE, '<', $current_file or die "cannot open file 
+$current_file: $!";
     # ...