That would be the easiest Pure-Perl way of doing it:

open(FILE,"/path/to/file.txt") or die "could not open file: $!";
my @_lines = <FILE>; # After I open the file for reading
close(FILE);

if ( scalar(@_lines) >= 100 ) {
  
  shift( @_lines ) for ( 0 .. (scalar(@_lines) - 100) );
  # shift removes the first element of the list
  # but what if you had 110 lines? you have to loop
  # and you start the loop at 0 because if you have 100 lines
  # 100 - 100 = 0 and 0 .. 0 will give you one iteration
  # but 1 .. 0 would not loop at all
  
  # then you overwrite the file
  open(FILE,">/path/to/file.txt") or die "could not open file: $!";
  print FILE @_lines;
  close FILE;
}
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