In one of the cases you forgot the *

Interestingly, I don't get any difference in SHA1 (with or without the * — though the latter doesn't work with strictures of course, i.e. it gives Bareword "IN" not allowed while "strict subs" in use).

#!/usr/bin/perl -l

#use strict;
use warnings;
use Digest::SHA1;

my $fname = $ARGV[0];
my $sha1;

open(IN, $fname) || die "can't open '$fname' for input: $!";
binmode IN;
$sha1 = Digest::SHA1->new;
$sha1->addfile(IN);
print $sha1->hexdigest;

open(IN, $fname) || die "can't open '$fname' for input: $!";
binmode IN;
$sha1 = Digest::SHA1->new;
$sha1->addfile(*IN);
print $sha1->hexdigest;

open(my $fhIN, $fname) || die "can't open '$fname' for input: $!";
binmode $fhIN;
$sha1 = Digest::SHA1->new;
$sha1->addfile($fhIN);
print $sha1->hexdigest;

system 'sha1sum', $fname;
[download]

Output for a file "foo" containing "bar\n":

$ ./840698.pl foo
e242ed3bffccdf271b7fbaf34ed72d089537b42f
e242ed3bffccdf271b7fbaf34ed72d089537b42f
e242ed3bffccdf271b7fbaf34ed72d089537b42f
e242ed3bffccdf271b7fbaf34ed72d089537b42f  foo
[download]

Anyhow, to avoid the confusion, the OP could've used a lexical file handle, as shown in the third variant above.