my $string = '...blah...<file fiop="foo" length="bar"/>baz</file>...blah...';

my ( $foo, $bar, $baz );
if ( $string =~ s{<file fiop="([^"]+)" length="([^"]+)"/>([^<]+)</file>}{} ) {
    ( $foo, $bar, $baz ) = ( $1, $2, $3 );
    print "extracted $foo, $bar, $baz; left $string\n";
}