It's hard to judge from your incomplete code excerpt what went wrong.
It partly works for me though.

Some remarks:

#build a dynamic sql statement
my $Sql_str = &BuildSQLString($cMssFactBag);
print $Sql_str;
#I print here I get no SQL statement
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So far, it looks not too bad, except that you shouldn't be using an ampersand (&) here, unless you know how it alters the behaviour of the subroutine called (circumvents prototypes; without arguments being called, also passes @_ implicitly)

sub BuildSQLString
{
...
$dynam_Sql_stmt = join(" ",$dynam_Sql_stmt, "from $p_str_tblName where
+ RECORDXFEREDTOFILE = 'I'");

#I print here I a SQL statement

return $dynam_Sql_stmt;

}
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Where do $p_str_tblName, $dynam_Sql_stmt come into life? May we see what ... stands for?

A few related hints:

The scoping of some variables is inadequate (see my, local & our)

The arguments weren't properly passed to or received by the subroutine (see perlsub)

There's a spelling error lurking somewhere (can be caught by strict)

However, I recommend to enable strict & warnings in most circumstances.

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