Capture part of URI and do not redirect.

I think you can just replace:

[HTTP::uri] starts_with "/abc"

with:

[HTTP::path] equals "/abc"

I think you need to use (not starts_with) operator for your logic. 

In the below code sample, if the request equals to "abc" or not starts with "abc-def", it will be redirected to /xyz.

Therefore, 

"If the uri is /abc (and whatever parameters after /abc, eg: /abc?=un), it should redirct to /xyz
If the uri is /abc-def (and whatever parameters after /abc-def, eg: /abc-def?=un, it should stay on same URI /abc-def"

• /abc will be redirected to /xyz
• /abc?=un will be redirected to /xyz (because it is not starts with "abc-def"
• /abc-def will stay on same because it will not going into second if statement.

when HTTP_REQUEST {
    if {[HTTP::host] eq "www.example.com"}{
        if{(([HTTP::uri] eq "abc") or !([HTTP::uri] starts_with "abc-def"))  }{
        HTTP::redirect "www.example.com/xyz"
    }
    }
}

i suggest you use gui based local traffic policy rather than irules scripting for this traffic modification

https://community.f5.com/t5/technical-articles/to-irule-or-not-to-irule-introduction-to-local-traffic-policies/ta-p/279536

when HTTP_REQUEST {
    set HOST [string tolower [HTTP::host]]
    set URI [HTTP::uri]

    if { $HOST equals "example.com" } {
        if { $URI equals "/abc-def?=un" } {
            HTTP::redirect "/abc-def"
            return
        } elseif { $URI starts_with "/abc" && !($URI contains "abc-def") } {
            HTTP::redirect "/xyz"
        }
    }
}