You just have to make parts of the regex optional, and ensure you capture the right parts.To get the captured values as return values, you need to use the /g flag.

#!/usr/bin/perl

while( <DATA> )
    {
    chomp;
    
    my( $year, $julian, $hour, $minute, $second ) = m/
        (\d{4})         # year
        (?:               # group the day - time portions
            -?          # optional hyphen
            (\d{1,3})   # julian day
            (?:           # group the time portions
                \s+     # one or more whitespace
                (\d\d)  # hour
                :
                (\d\d)  # minute
                :
                (\d\d)  # second
            )?            # time is optional
        )?                # day - time is optional
        /xg;
        
    print <<"HERE";
For input [$_]
Year:   $year
Julian: $julian
Hour:   $hour
Minute: $minute
Second: $second

HERE

    }
    
__END__
2005-100 10:12:01
2005100 10:12:01
2005-100
2005100
2005
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