Thanks for the update! Sometimes calling the native tools turns out to be the "easiest" way to go, in cases like that I usually make sure the interface is as clean as possible.

I've not actually integrated into my scripts, but as a proof of concept, it works.

A quick test shows me that something like this seems to work:

use IPC::Run3 'run3';
run3 ['powershell', '-Command', 'Invoke-WebRequest', '-Uri', $uri,
    '-OutFile', $filename, '-UseDefaultCredentials'];
$? and die "run3 failed with \$?=$?";
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(I'm using IPC::Run3 because newer versions use Win32::ShellQuote under the hood, and it allows redirection of STDIN/OUT/ERR if desired. For really complicated cases, I guess PowerShell's -EncodedCommand is probably the way to go.)

Update: Wrapped up in a function:

use File::Temp qw/tempfile/;
use IPC::Run3 'run3';
sub invoke_webrequest {
    my $uri = shift;
    my ($tfh,$tfn) = tempfile(UNLINK=>1);
    close $tfh;
    run3 ['powershell', '-Command', 'Invoke-WebRequest', '-Uri', $uri,
        '-OutFile', $tfn, '-UseDefaultCredentials'];
    $? and die "run3 failed with \$?=$?";
    open my $fh, '<', $tfn or die "$tfn: $!";
    my $data = do { local $/; <$fh> };
    close $fh;
    unlink $tfn;
    return $data;
}
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