• \xEF\xBB\xBF<?xml version="1.0" encoding="UTF-8"?>
• <?xml version="1.0" encoding="utf-8"?>
• <?xml version='1.0' encoding='UTF-8'?>
• <?xml version="1.0"?>
• <?xml encoding="UTF-8" version="1.0"?>
• <?xml version="&#x31;&#x2E;&#x30;" encoding="&#x55;&#x54;&#x46;&#x2D;&#x38;"?>

And that's assuming you're just supporting UTF-8!

This isn't the job of a regex. But if you insist on using a regex match, here you go:

use XML::LibXML qw( );

local our $doc;
$content =~ /(?(?{ !eval {
    $doc = XML::LibXML->new()->parse_string($_)
} })(?!))/;
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The match will succeed if the parsing succeeds (and set $doc to a XML::LibXML::Document object) and fail if the parsing fails.

Even for something as seemingly simple as that, you should still reach for a parser. You can certainly make do with regular expressions but a parser buys you much more, will fall down less, is easier to write and read, and won't give you false positives. Your sample XML, for example, is broken but your regex wouldn't tell you that.

use strict;
use warnings;
use XML::LibXML;

my $doc = XML::LibXML->new->parse_fh(\*DATA);

print
    "All about me-\n",
    "      Version: ", $doc->version, "\n",
    "     Encoding: ", $doc->encoding, "\n",
    "   Standalone: ", $doc->standalone, "\n";

__DATA__
<?xml version="1.0" encoding="UTF-8"?><sample></sample>
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All about me-
      Version: 1.0
     Encoding: UTF-8
   Standalone: -2
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$content = qq(?XML version="1.0');

if ($content =~/\?XML\s+version=[\'\"]1.0[\'\"](\s+encoding=[\'\"][^\'
+ +\"]*[\'\"])?/){ 
    print"Input file is valid";
}else{ 
    print"File is invalid";
}
__END__
Input file is valid
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Maybe you want to look at perlop for string equality, or maybe you want to use a real XML parser?

Your RE contains XML, the string you want to match contains xml and you haven't used the i (ignore case) modifier (see perlre).