Don't split one value into 4 variables, only one of which can be true.

Don't include $! in your error message. You didn't do any system calls that set $!.

Why aren't you using die? It'll be much more simpler and reliable to catch exceptions and/or STDERR rather than using errmsg everywhere.

sub inspect {
    return "windows"
        if $^O eq 'MSWin32' || $^O eq 'Windows_NT';

    my $frunext = $ENV{FRUNEXT}
        or die("Environment variable 'FRUNEXT' not set\n");

    return $frunext
       if $frunext =~ /^(?:ifx|msv|ora)$/;

    return '';
}
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Or if you want to do some translation:

my %db_lkup = (
    ifx => 'informix',
    msv => 'sql',
    ifx => 'oracle',
);

sub inspect {
    return "windows"
        if $^O eq 'MSWin32' || $^O eq 'Windows_NT';

    my $frunext = $ENV{FRUNEXT}
        or die("Environment variable 'FRUNEXT' not set\n");

    return $db_lkup{$frunext}
       if exists( $db_lkup{$frunext} );

    return '';
}
[download]

I didn't know if you meant for an unrecognized value of $ENV{FRUNEXT} to be an error. It wasn't, so I didn't change that.