As an aside, you may want to look at Querylet, which implements the "run SQL, create Excel" workflow.

If you still want to roll your own, have a look at various templating engines, Interpolation or this very, very basic regex:

my %variables= (
    table_name => 'EMPLOYEE', # setting a table name
);

...

$sql=~ s!\$(\w+)!$variables{ $1 } || "\$$1" !ge;
print $sql;
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