@l = ...;

m[
    <MIR-\d+> 
    (?:<EXP-V-\d+>)? 
    (?:<ASSC-PHRASE-\d+>|<ART-\d+>)? 
    (?:<BE-V>)? 
    <VACCVIRUS-PROP-\d+> 
    (?:<PATTERN-\d+>)?
]x  and print for @l;;
<MIR-1><EXP-V-3><VACCVIRUS-PROP-1>
<MIR-1><ASSC-PHRASE-1><VACCVIRUS-PROP-1><PATTERN-1>
<MIR-1><EXP-V-0><ART-0><VACCVIRUS-PROP-1>
<MIR-1><EXP-V-0><ART-0><BE-V><VACCVIRUS-PROP-1>