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my $Usage = "Usage: $0  file1  file2 > combined.file\n";
die $Usage unless ( @ARGV == 2 and -f $ARGV[0] and -f $ARGV[1] );

my ($inf1,$inf2) = @ARGV;

open( IN1, $inf1 ) or die "Can't open $inf1: $!";

my %file1;

...
    $file1{$key} = $val;
}
close IN1;

open( IN2, $inf2 ) or die "Can't open $inf2: $!";
while (<IN2>) {
...
    $file1{$key} .= ";$val";
}
close IN2;

  perl_script  infile.a infile.b > outfile  2> script.errs