use strict;
use warnings;
use File::Copy;

my $infilename = 'Pull-2015-08-15-23.txt';
my $outfilename = 'new.file';
my $find = '20150518';
my $replace = '18 May 2015';

open $infile,'<',$infilename;
open $outfile,'>',$outfilename;
while (my $line = <$infile>) {
  $line =~ s/$find/$replace/;
  print $outfile $line;
}
close $infile;
close $outfile;
move($outfilename,$infilename);