A couple of things. First and foremost, don't you think that ought to be:

my $slurp = join $/, <FH>;
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Otherwise you can't really distinguish between lines. Next, you're having perl read the file in, split the file on $/, and then rejoin everything and you end up with a string that was exactly what it was before the split. If you just localize $/ and then read the file in, you're done. Of course, I have some nice benchmarks that show the local $/ method to be a little under 5 times faster. As with any other benchmarks, YMMV.

#!/usr/bin/perl -w

use Benchmark qw(cmpthese);

# file.txt is about 2200 lines each between 5 and 50 chars long
open FH, "file.txt" or die $!;

sub scalarcon {
  seek(FH,0,0);
  local $/;
  <FH>;
}

sub listjoin {
  seek(FH,0,0);
  join '',<FH>;
}

cmpthese(-5,{scalarcon=>\&scalarcon,listjoin=>\&listjoin});
__DATA__
Benchmark: running listjoin, scalarcon for at least 5 CPU seconds...
  listjoin:  5 wallclock secs ( 5.25 usr +  0.12 sys =  5.38 CPU) @ 30
+.70/s (n=165)
 scalarcon:  6 wallclock secs ( 1.95 usr +  3.36 sys =  5.30 CPU) @ 15
+3.26/s (n=813)
            Rate  listjoin scalarcon
listjoin  30.7/s        --      -80%
scalarcon  153/s      399%        --
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Update: As chromatic has pointed out, I'm a twit.

Hope this helps.

antirice    
The first rule of Perl club is - use Perl
The ith rule of Perl club is - follow rule i - 1 for i > 1