sub testfunc (\%)
{
        my ( $href ) = @_;

        my $key = "file4";
        my $insert_val="host1";
        my $insert_val2="foo";

        #push(@{$href{$key}}, $insert_val);
        #push(@{$href{$key}}, $insert_val2);


        foreach my $file (keys %$href) {
            print "The content of $file is:\n";
            foreach my $line ( $href->{$file} ) {
                print "\t$line\n";
            }
        }
}

my %hash = ( file1 => [ 'host1', 'bla', 'foo', ], file2 => [ 'host2', 'host1', '..', ], file3 => [ 'x', 'host2', 'host1' ] );
testfunc(%hash);