open my $fh, '<', 'hash.txt' or die "Can't open file $!";
LINE:
while (my $line = <$fh>)
     {
     my ($key, $value) = split /\s/, $line;
     next LINE if not $key;
     $hash{$key} = $value;
     chomp (%hash);
     }
close $fh;

@file_array =<*.txt> or die $!; 
foreach $file (@file_array)
     {
     open FH, "$file", or die $!;
     while (<FH>)
          {
          if (/(\S+):(\S+).*\n/)
               {
               s/$1/$hash{'$1'}_$2/g;
               }
           }
     close FH;
      }