Perl's I/O is buffered, so it does one I/O for every disk block regardless of which method you use.

#!/usr/bin/perl

if ($ARGV[0] eq 'line')
{
  print "Line-at-a-time\n";
  while (<STDIN>)
  {
    print
      if /perl/;
  }
}
else
{
  print "All at once\n";
  my @arr = (<STDIN>);
  foreach (@arr)
  {
    print
      if /perl/;
  }
}
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On my system, the block size is 4096 bytes. On an 8K file with 128 lines, we see:

$ strace -e read /tmp/t29 line </tmp/t29.8192 >/dev/null
...
read(0, "This is a line that contains the"..., 4096) = 4096
read(0, "This is a line that contains the"..., 4096) = 4096
read(0, "", 4096)                       = 0

$ strace -e read /tmp/t29 slurp </tmp/t29.8192 >/dev/null
...
read(0, "This is a line that contains the"..., 4096) = 4096
read(0, "This is a line that contains the"..., 4096) = 4096
read(0, "", 4096)                       = 0

Still, the diamond operator takes some time to operate, so slurping is still probably faster, but not because of I/O.