So my question is that why @array is behaving like a static variable in C language here?

It's an artifact of the implementation (and in particular, with an optimization). my has compile-time and run-time effects, and due to the if statement, not all run-time effects happen. As pointed out before, don't rely on it - it may not work like this in the future. And it may not work as expected now either - it won't act like a static C variable if you use recursion:

#!/usr/bin/perl

use 5.010;


use strict;
use warnings;

sub foo {
    my $x if 0;
    say ++$x;
}

sub bar {
    my $x if 0;
    say ++$x;
    bar() if @_;
}

foo;
foo;
bar(1);

__END__
Deprecated use of my() in false conditional at ./x line 10.
Deprecated use of my() in false conditional at ./x line 15.
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Note that the second time bar() is called (due to recursion), you do get a different $x - unlike the foo() case.

Since 5.10, there is a warning on my $foo if 0; (at compile time). People have argued there should always be a warning on any my $foo if EXPR or my $foo = EXPR1 if EXPR2; usage, but that hasn't happened, and isn't happening in blead either.

Also since 5.10, if you want a static variable, you can use the state keyword.