• "my" variable $lo masks earlier declaration in same scope at foo.pl li
  +ne 14.
  "my" variable $hi masks earlier declaration in same scope at foo.pl li
  +ne 14.
  "my" variable @array masks earlier declaration in same scope at foo.pl
  + line 21.
  "my" variable $try masks earlier declaration in same scope at foo.pl l
  +ine 21.
  "my" variable $word masks earlier declaration in same scope at foo.pl 
  +line 21.
  "my" variable @a masks earlier declaration in same scope at foo.pl lin
  +e 33.
  syntax error at foo.pl line 12, near "<= :"
  syntax error at foo.pl line 30, near "}"
  Execution of foo.pl aborted due to compilation errors.
  [download]

  Untested, right? After removing the obvious vi command in line 12, I still get

  "my" variable @a masks earlier declaration in same scope at foo.pl lin
  +e 32.
  [download]

  Please don't post untested code.

• use lib '/home/kaiyin/perl5/mylib'; Why? You don't load any non-core modules.
• Why do you pass the searched word by reference?
• Why don't you allow numeric compare?
• Why don't you place your code in a reusable, testable module?
• And finally, why don't you just use grep? At least with your trivial example, it is significantly faster.

I modified your code slightly, to show the speed differences.

Relevant NYTProf output:

Line	State ments	Time on line	Calls	Time in subs	Code
37	1	12µs			for (1..100000) {
38	100000	928ms	100000	4.23s	$result=BinSearch(\@a, \$w); # spent 4.23s making 100000 calls to main::BinSearch, avg 42µs/call
39	100000	1.46s	100000	728ms	say $result; # spent 728ms making 100000 calls to main::CORE:say, avg 7µs/call
40	100000	1.43s			($result)=grep { $a$_ eq $w } 0..$#a;
41	100000	1.65s	100000	673ms	say $result; # spent 673ms making 100000 calls to main::CORE:say, avg 7µs/call
42					}

Modified code:

my @a = qw(format type ascii hex pos len binary search perl unix eof a
+rray word);
@a = sort @a;
my $w = "len";
say "@a";
my $result;
open STDOUT,'>','/dev/null';
for (1..100000) {
    $result=BinSearch(\@a, \$w);
    say $result;
    ($result)=grep { $a[$_] eq $w } 0..$#a;
    say $result;
}
[download]

Even with 10,000 pieces of junk in front of the searched element, grep still beats your BinSearch easily:

Relevant NYTProf output:

Line	State ments	Time on line	Calls	Time in subs	Code
38	1	1.66ms			for (1..1000) {
39	1000	24.1ms	1000	14.4s	$result=BinSearch(\@a, \$w); # spent 14.4s making 1000 calls to main::BinSearch, avg 14.4ms/call
40	1000	46.9ms	1000	34.3ms	say $result; # spent 34.3ms making 1000 calls to main::CORE:say, avg 34µs/call
41	1000	6.35s			($result)=grep { $a$_ eq $w } 0..$#a;
42	1000	62.3ms	1000	29.4ms	say $result; # spent 29.4ms making 1000 calls to main::CORE:say, avg 29µs/call

Modified code:

my @a = ('00junk') x 10000;
push @a,qw(format type ascii hex pos len binary search perl unix eof a
+rray word);
@a = sort @a;
my $w = "len";
say "@a";
my $result;
open STDOUT,'>','/dev/null';
for (1..1000) {
    $result=BinSearch(\@a, \$w);
    say $result;
    ($result)=grep { $a[$_] eq $w } 0..$#a;
    say $result;
}
[download]

Alexander