This works:

while ( <DATA> )
{
    $_ =~ s/( CHECK\(.*,)/,\n   \1/;
    #Find a space followed by a
    #CHECK followed by a (
    #and then followed by anything but
    #having to be followed by a ,
    #and substituting with comma a
    #new line and then anything that was
    #matched previously between the parens
    #the () not the \()
    print $_;
}


__DATA__

CREATE TABLE "ARSAMSUSER"."LINK_TYPE_CODE"
(
    LINK_TYPE             VARCHAR2(20) NOT NULL,
    DCD_NO                VARCHAR2(1) NULL,
    IS_ONE_TO_ONE         VARCHAR2(1) NULL CHECK(IS_ONE_TO_ONE IS NULL
+ OR(IS_ONE_TO_ONE IN(Y,N,NULL))),
    IS_DRAWN              VARCHAR2(1) NULL CHECK(IS_DRAWN IS NULL OR(I
+S_DRAWN IN(Y,N,NULL))),
    DESCRIPTION           VARCHAR2(60) NULL,
    LENGTH_OPERATION          VARCHAR2(4) NULL,
    WIDTH_OPERATION           VARCHAR2(4) NULL,
    HEIGHT_OPERATION          VARCHAR2(4) NULL,
    WEIGHT_OPERATION          VARCHAR2(4) NULL,
    AREA_OPERATION            VARCHAR2(4) NULL,
    PRIMARY KEY (LINK_TYPE)
)
TABLESPACE ARSAMS
/
[download]

But I am making assumptions based on this one piece of data. all s/CHECK\b//i does is check for the CHECK (case insensitive)followed by word boundary and then replace it with nothing (as you mentioned).