You can extract both, the key and the value in one go. Maybe you want some sanity checking on the key or maybe not:

if ($param) {
    $param =~ /^(id|key|limit):(\S*)$/ or die "Malformed value for par
+ameter: '$param'";
    my ($key,$value) = ($1,$2);
    $ENV{"C:$key"} = $value;
};
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If you don't want the sanity check on the parameters, for easy extensibility, it gets even shorter:

if ($param) {
    $param =~ /^(\w+):(\S*)$/ or die "Malformed value for parameter: '
+$param'";
    my ($key,$value) = ($1,$2);
    $ENV{"C:$key"} = $value;
};
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