As the input file is sorted, uniq infile > outfile ought to do the job very quickly.

If for some reason you don't have the uniq command available, try

#! perl -w
use strict;

my $last = <>;
while( <> ) {
    print $last if $_ ne $last;
    $last = $_;
}

__END__
thisscript infile  > outfile
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