use strict;
use XML::Rules;

my $parser = XML::Rules->new(
	stripspaces => 7,
	rules => {
		_default => sub {
			my ($tag,$attrs,$context) = @_;
			if (%$attrs) {
				my @tags = (@$context[1..$#$context], $tag);
				my $content = delete $attrs->{_content};
				if (%$attrs) {
					foreach my $attr (keys %$attrs) {
						$tags[-1] .= " $attr=\"$attrs->{$attr}\"";
					}
				}
				if (defined $content) {
					print join('::',@tags),"::",$content,"\n";
				} else {
					print join('::',@tags),"\n";
				}
			}
			return;
		}
	}
);
$parser->parse(\*DATA);

__DATA__
<FinInstnCdtTrf>
<GrpHdr>
<MsgId>0000003714</MsgId>
...