use strict;
use File::Copy;

my $dest_file = "email.lst";
my $searchstr = 'blue';
my $repstr    = 'foo';

open(FL, $dest_file) or die("Doh - $!");

# turn on inplace edit flag (see. man perlvar)
$^I = '~';
s/$searchstr/$repstr/g while <FL>;

close(FL);

move($dest_file, $repstr);