Use qr// to precompile a regex into a form that can be used instead of a regex on the right-hand-side of =~ and also interpolated into other regexen. Use named capture groups and %+ instead of $1..$9. Use (DEFINE) to reduce repitition within a single regex.

You didn't provide any sample input to really write a test program against, but here's a quick-and-dirty attempt to rewrite your regex that at least compiles. Quite a few more simplifications are probably possible, this is just to get you started.

my $regex = qr{
    (?<intplus> (?&INT) (?&SOMETHING)+ (?&QUOT)? )
    | (?<intonly> (?&INT) (?&QUOT)? )
    | (?<regonly> \b(?:E?A[XHL])=[0-9A-Fa-f]{2,}[Hh]?
        (?&SOMETHING)* (?&QUOT)? )
    | (?<table> \#[0-9A-Z][0-9]{4}\b)
    | (?<mem_16_16> \bMEM\s?(?&HEX4):(?&HEX4) (?&QUOT)? )
    | (?<mem_32> \bMEM\s?[0-9A-Fa-fXx]{1,8}[Hh]? (?&QUOT)? )
    | (?<call> \@(?&HEX4):(?&HEX4) (?&QUOT)? )
    | (?<portrange> \bPORT\s?(?&HEX4)-(?&HEX4) (?&QUOT)? )
    | (?<portsingle> \bPORT\s?(?&HEX4) (?&QUOT)? )
    (?(DEFINE)
        (?<INT> \bINT\s?[0-9A-Fa-f]{2}[Hh]? )
        (?<HEX4> [0-9A-Fa-fXx]{1,4}[Hh]? )
        (?<QUOT> (?:\"[^"]+\") )
        (?<SOMETHING>
            \/(?:E?[ABCD][XHL]|E?[SD]I|E?[SB]P|[DESC]S)=[0-9A-Fa-f]{2,
+}[Hh]? )
    )
}x;
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