Here is one way:

#!/usr/bin/perl
my @l = split /(\n)/, <<ENDE;
Guido bla1 ...
Mike  bla2 ...
Klaus bla3 ...
ENDE

my %h = qw/
Guido Meyer
Mike Smith
Klaus Rothschild/;

for (@l) {
 s/(\w+)/$h{$1} || $1/e;
 print
}
__OUTPUT__
Meyer bla1 ...
Smith  bla2 ...
Rothschild bla3 ...
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