Something like:

use strict;
use warnings;
use XML::Twig;

my $xmlStr = <<XML;
<doc>
    <testcase1>
    <step1 command ="Launchapp " param1 = "executablepath" param2 = "d
+irpath"/>
    <step2 command = "Dothis"/>
    <step3 command = "Changemode" param1 = "100"/>
    <step4 command = "CaptureRectanlge" param1 = "3" param2 = "96" par
+am3 = "726" param4 = "580"/>
    <step5 command = "CompareImage" image1 = "D:\\img1.jpg" image2 = "
+D:\\img2.jpg "/>
    </testcase1>

    <testcase2>
    <step1 command ="Launchapp " param1 = "executablepath" param2 = "d
+irpath"/>
    <step2 command = "Dothis"/>
    <step3 command = "Changemode" param1 = "100"/>
    <step4 command = "CaptureRectanlge" param1 = "3" param2 = "96" par
+am3 = "726" param4 = "580"/>
    <step5 command = "CompareImage" image1 = "D:\\img1.jpg" image2 = "
+D:\\img2.jpg "/>
    </testcase2>
</doc>
XML


my $twig = new XML::Twig(twig_handlers => {'*' => \&testcase});

$twig->parse($xmlStr);


sub testcase {
    my ($twig, $testcase) = @_;
    my $tag = $testcase->tag();

    return 1 if $tag !~ /^testcase\d+$/;

    print "$tag\n";

    for my $step ($testcase->children()) {
        my $tag = $step->tag();
        my %attrs = %{$step->atts()};
        print "Command: $attrs{'command'}(";
        print join ', ', map {$attrs{$_}} grep {/^param/} sort keys %a
+ttrs;
        print ")\n";
    }

    print "\n";
}
[download]

Prints:

testcase1
Command: Launchapp (executablepath, dirpath)
Command: Dothis()
Command: Changemode(100)
Command: CaptureRectanlge(3, 96, 726, 580)
Command: CompareImage()

testcase2
Command: Launchapp (executablepath, dirpath)
Command: Dothis()
Command: Changemode(100)
Command: CaptureRectanlge(3, 96, 726, 580)
Command: CompareImage()
[download]

This is somewhat messier than it ought be because the test case tags are all different. If the tags were 'testcase' and the case identifier were an attribute the twig_handlers expression could be just 'testcase' and the testing in the handler sub is no longer required.