I wonder is there any way I can write Perl program, which would use any inbuilt function to create 'b.txt' on Windows itself.

In the spirit of TIMTOWTDI!:

#!/usr/bin/perl -w 
use strict;
ues Tie::File;

my @ry=();
tie @ry,"Tie::File","a.txt" or die $!;
my %een=();
@ry = grep !$een{$_}++,sort @ry;
untie @ry;
[download]

Sort of an implementation of an inline unique sort.

HTH