/path/to/some/file

##</code><code>##

#!/usr/bin/env perl -l

use strict;
use warnings;

my $re = qr{file:(.*?).xml};

while (<DATA>) {
    print $1 if /$re/;
}

__DATA__
<xml>
<info>
<file>file:/path/to/someA/file.mxf</file>
</info>
<info>
<file>file:/path/to/someB/file.xml</file>
</info>
</xml>

##</code><code>##

/path/to/someB/file

##</code><code>##

#!/usr/bin/env perl -l

use strict;
use warnings;

my $re = qr{file:(.*?\.xml)};

while (<DATA>) {
    print $1 if /$re/;
}

__DATA__
<xml>
<info>
<file>file:/path/to/someA/file.mxf</file>
</info>
<info>
<file>file:/path/to/someB/file.xml</file>
</info>
</xml>

##</code><code>##

/path/to/someB/file.xml