$href is empty, since it is not passed into foo. hence @array ends up being a package variable and not a lexical variable.
your code

sub foo{
        my $var = shift;
        my $href = shift;
        my @array = $href->{pass_in} if defined $href and exists $href
+->{pass_in};
        push @array, $var;

        print "contents : @array\n";
}

foo(1);
foo(2);
[download]

This gives
contents : 1
contents : 1 2

However, commenting out the unused href code and declaring array as a lexical like:

sub foo{
        my $var = shift;
        my @array; 
        #my $href = shift;
        #my @array = $href->{pass_in} if defined $href and exists $hre
+f->{pass_in};
        push @array, $var;

        print "contents : @array\n";
}

foo(1);
foo(2);
[download]

gives

contents : 1
contents : 2

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.