I have to say that I hate your use of the trinary operator there. You're expressing that you want to assign die's result to $file. Instead:

my $file = $ARGV[0] or die "Usage: prog.pl file_pathname\n";
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(as a bonus, you don't have the redundant mention of $ARGV[0]).
Better, if the filename might be zero:

@ARGV or die "Usage: prog.pl file_pathname\n";
my $file = $ARGV[0];
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