in first case, when exactly perl runs subroutine? does it runs get_vlaue2() every time we try to access $hash->{key}?

You can use a few print statements to find this out:

print "Start\n";
my $hash = { key => 'value1' . get_value() };
print "hash created\n";
print "accessing hash key\n";
my $key = $hash->{'key'};
print "hash key is '$key'\n";
print "Finish\n";

sub get_value {
    print "get_value() called\n";
    return "value";
}
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