Sorry, I can't help you directly with your issue at hand - your program works fine for me, even on bigger files. But allow me to give two remarks:

• @lines = {}; assigns an empty hashreference to the array @lines. What you want is probably @lines = (); (with parentheses and not curly braces) which clears the array.
• Your sub is written in a strange way. Normally you would return a reference to a lexically created array like this - instead of passing in an arrayref and modifiying that:

  sub myfile
  {
    my $file = shift;
    open my $fh, $file or die "Couldn't open '$file': $!";   
    my @lines = <$fh>;
    close $fh or die "Couldn't close '$file': $!";
    return \@lines;
  }
  
  # and call it like this:
  my @lines = @{myfile('xml.log')};
  [download]

Update: If it's a bug as demerphq suspects, then I suspect that it's OS dependent - I get the same times here on all opens (perl v5.6.0 built for sun4solaris).

-- Hofmator