how to avoid pushing duplicate values into an array in perl

You simply need to use hash like this:

my %hash;
$hash{$key} = $value;  # you can use 1 as $value
...

This will automatically overwrite duplicate keys.

When you need to print it, simply use:

foreach my $key (keys %hash) {
    # do something with $key
}

If you need to sort keys, use

foreach my $key (sort keys %hash) ...

push(@yourarray, $yourvalue) unless grep{$_ == $yourvalue} @yourarray;

This checks if the value is present in the array before pushing. If the value is not present it will be pushed.

If the value is not numeric you should use eq instead of ==.

by using ~~ we can minimum perl version is 5.10.1

    use v5.10.1;
use strict;
use warnings;

my @ARRAY1 = qw/This is array of backup /;
my @ARRAY2;


my $value = "version.xml" ;


sub CheckPush($$)
{

my $val = shift (@_);
my $array_ref= shift (@_);

     unless ($val ~~ @$array_ref )
     {
        print "$val is going to push to array  \n";
        push(@$array_ref,$val);
     }   
return (@$array_ref);
} 

@ARRAY1 = CheckPush($value,\@ARRAY1);
print "out \n";
foreach $_ (@ARRAY1) {
print "$_ \n";
}

@ARRAY2 = CheckPush ($value,\@ARRAY2);
print "out2 \n";
foreach $_ (@ARRAY2) {
print "$_ \n";
}