Thanx for the replies.......
Solved the problem by using current directory......

#!/usr/bin/perl -w
my ($dir,$file,$old,$new);
print "Enter filename: ";
$file = <STDIN>;
print "Enter the name of output file: ";
$out = <STDIN>;
print "Enter old pattern: ";
$old = <STDIN>;
print "Enter new pattern: ";
$new = <STDIN>;
chomp ($file,$old,$new, $out);
open (OUT, ">>$out") || die $!;
opendir (DIR, ".") || die $!;
my @files = readdir(DIR);
open (FH, "$file") || die "cant open file $!";
my @lines = <FH>;
foreach my $line (@lines)
{
if (grep /$old/,$line)
{$line =~ s/$old\s/$new/g;}
print OUT $line;
}
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